The present invention relates to an improvement in a zinc-chloride cell in which the cell internal pressure in operation can be easily controlled and the chlorine concentration in an electrolyte during charging can be reduced.
In a zinc-chloride cell, chlorine is generated from a positive electrode (chlorine electrode) and zinc electrically deposits on a negative electrode (zinc electrode) during charging. Chlorine generated at the positive electrode is mixed with cooled water and is stored in the form of a solid, i.e., a chlorine hydrate. For discharging, the chlorine hydrate is heated and decomposed (at a decomposition temperature of 9.6.degree. C. at normal pressure), dissolved in the electrolyte and supplied to the positive electrode.
The reaction of producing a chlorine hydrate from chlorine and cooled water is expressed by: EQU Cl.sub.2 +xH.sub.2 O.revreaction.Cl.sub.2.xH.sub.2 O(x: 6 to 8)
The volume change after the reaction can be given by: EQU (6.times.8.times.1.1)/(22.4.times.10.sup.3 +6.times.18)=1/189
for x=6, and EQU (8.times.18.times.1.1)/(22.4.times.10.sup.3 +8.times.18)=1/142
for x=8
Thus, the volume is considerably reduced upon this reaction.
A zinc-chloride cell is conventionally assembled in a sealed container, and the gas phase in the container consists of 100% chlorine. For this reason, when the hydrate decomposes, a change in internal pressure upon a change in volume of the container contents is large, and pressure control is difficult.
In addition, when the gas phase in the cell is 100% chlorine as in a conventional zinc-chloride cell, the chlorine concentration dissolved in the electrolyte is high, thus accelerating self corrosion of the zinc electrode and causing a large decrease in current efficiency. In order to prevent this, the dissolved chlorine concentration is conventionally decreased by various methods. In one method, a strong electrolyte such as sodium chloride or potassium chloride is added to the electrolyte or the temperature of the electrolyte is increased to decrease the solubility of chlorine in the electrolyte. Alternatively, the interior of the container is reduced in pressure or the chlorine partial pressure in the gas phase in the container is decreased to reduce the dissolved chlorine concentration.
However, a satisfactory effect is not obtained if only a strong electrolyte is added to the electrolyte or the temperature of the electrolyte is increased. In addition, when an excessive amount of a strong electrolyte is added or the temperature of the electrolyte is excessively increased, dendritic deposition at the zinc electrode is accelerated to cause short-circuiting with the chlorine electrode. In order to keep the interior of the container at a reduced pressure, the container must have a predetermined pressure resistance, resulting in problems with respect to size, weight and cost of the cell. Even if the chlorine partial pressure in the gas phase of the container is decreased, since the chlorine gas is dissolved in the electrolyte in a supersaturation state, the chlorine concentration in the electrolyte cannot be sufficiently reduced.